Admin Notes for Groups

What does this MR do?

This merge request adds the option for admins to leave a note for each group, visible to only other admins. See #26468 (closed) for the discussion.

Closes #26468 (closed)

Screenshots (strongly suggested)

Administrative notes for a group will only be visible to the instance administrators, so they are only available in the Admin area.

An admin can create a new administrative note for a group when creating the group in the Admin Area (Admin Area -> Groups -> New group):

Then it will be visible in the group's Admin page:

An existing group may not have an administrative note. For these groups, no admin note will show in the group's Admin page:

When pressing Edit, one can be added at the bottom of the page:

...and then it will be visible for all admins to see:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Database

Migration output:

$ bundle exec rake db:migrate:redo VERSION=20201218194311
WARNING: This version of GitLab depends on gitlab-shell 13.16.1, but you're running 13.14.0. Please update gitlab-shell.
== 20201218194311 CreateAdminNotes: reverting =================================
-- drop_table(:namespace_admin_notes)
   -> 0.0034s
== 20201218194311 CreateAdminNotes: reverted (0.0035s) ========================

== 20201218194311 CreateAdminNotes: migrating =================================
-- create_table(:namespace_admin_notes, {})
-- quote_column_name(:note)
   -> 0.0000s
   -> 0.0127s
-- quote_table_name("check_e9d2e71b5d")
   -> 0.0000s
-- quote_table_name(:namespace_admin_notes)
   -> 0.0000s
-- execute("ALTER TABLE \"namespace_admin_notes\"\nADD CONSTRAINT \"check_e9d2e71b5d\" CHECK (char_length(\"note\") <= 1000)\n")
   -> 0.0009s
== 20201218194311 CreateAdminNotes: migrated (0.0244s) ========================